dumb

slow to learn or understand; lacking intellectual acuity; "so dense he never understands anything I say to him"; "never met anyone quite so dim"; "although dull at classical learning, at mathematics he was uncommonly quick"- Thackeray; "dumb officials make some really dumb decisions"; "he was either normally stupid or being deliberately obtuse"; "worked with the slow students"

temporarily incapable of speaking; "struck dumb"; "speechless with shock"

lacking the power of human speech; "dumb animals"

unable to speak because of hereditary deafness